1. Field of Endeavor
This invention relates to a turbine blade, and further relates to a process for producing a turbine blade.
2. Brief Description of the Related Art
Turbine blades are subjected to very high temperatures of the hot fluid driving the turbine. In order to prevent damage to the blades due to the high temperatures and in order to assure a reasonable lifetime of the turbine, turbine blades are often cooled externally and internally by a cooling medium, typically by using cooling air bled from the compressor of the gas turbine. Internal cooling of the turbine blade is realized by several passages within the blade between the pressure sidewall and the suction sidewall of the turbine blade. The passages typically extend spanwise from the root of the blade to its tip. Some of the passages are formed of a single passage with an exit port near the tip of the blade and/or several film cooling holes on the edge or on the side wall of the blade. Other passages may follow a serpentine path allowing the cooling fluid to flow for example from the root to the tip and around a 180° turn. From the tip it extends towards the root and around a further 180° turn that directs it again toward the tip where it finally exits through exit ports or film cooling holes. Serpentine cooling passages of this type are disclosed, for example, in EP 670953. They allow for a high internal heat transfer with a minimum amount of cooling air.
A typical blade of the state of the art includes several internal passages extending radially inward and outward between a root section and a tip. A first internal passage extends from an entry opening in the root section radially outward to the tip of the blade. Cooling fluid can flow from the root section through the passage and exit via several cooling slots along the trailing edge as well as through a tip hole. A second internal passage extends from an entry opening radially outward along the leading edge of the blade. Cooling fluid flows through this passage and exits via a tip hole and through several rows of film cooling holes drilled through the leading edge of the blade. A serpentine passage includes an entry opening at the radially inner end of the root section, a first passage extending radially outward with a tip hole. At the tip a 180° turn leads to a passage extending radially inward. At the radially inner end of the passage a second 180° turn leads to a third passage extending radially outward to a tip hole. Cooling fluid flowing through the straight and serpentine passages cool the blade from within by impingement cooling and exits through the film cooling holes on the edges of the blade and/or through the tip holes. Other typical blades have several serpentine cooling passages or serpentine passages including five passages with four turns.
Blades with internal serpentine geometry for the cooling passages are typically manufactured by an investment casting process, which utilizes a ceramic core to define the individual internal passages. Following the casting the ceramic core is removed from the blade by a leaching process. The film cooling holes on the edges and sidewalls of the blade are then realized by a laser drilling process. This process involves, prior to the actual drilling, the insertion of a backing or blocking material which limits the laser radiation to the desired locations of the film cooling holes and prevents damage to the passage walls and other inner surfaces of the blade. Such a method is disclosed, for example, in EP 854005. It uses a wax material as a blocking material.
Another suitable drilling process could be an ion beam drilling process.
During the process of casting the internal passages it is often difficult to maintain the separation of the passages in the cores due to thermal strains caused by differential heating and cooling rates of the core and surrounding metal.
A current practice to maintain the separation of the serpentine passages and to support the core during the casting process utilizes conically shaped features in the core. These conical features are formed as part of the core and extend from the root section through an opening in the wall of the 180° turn and into the passages. After the part is cast and the core is leached out, the conical feature is closed off with a spherically shaped plug that is brazed into place, as described in EP 1267040.
Finally, a TBC (Thermal Barrier Coating) coating is applied to the turbine blade. This coating serves to insulate components from large and prolonged heat loads by utilizing materials with lower thermal conductivity which can sustain an appreciable temperature difference between the load bearing alloys and the coating surface. The thermal insulation system coatings often are formed of three layers: the metal substrate, metallic bond coat, and TBC ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ) which is desirable for having very low conductivity while remaining stable at nominal operating temperatures. This ceramic layer creates the largest thermal gradient of the thermal insulation system and keeps the lower layers at a lower temperature than the surface. Once applied to the turbine blade, subsequent welding and/or brazing is not feasible in an economical way.
EP 1267040 discloses an airfoil having internal cooling air passages arranged in a serpentine manner with one or more radially outward and radially inward extending passages. The passages are in fluid connection by turns of approximately 180°. According to that document, the turns near the platform of the airfoil connecting a radially inward extending passage with a radially outward extending passage is realized by a root turn defined by the passage sidewalls, which extend radially inward to the radially inner end of the root section of the airfoil, and by an end plate attached to the radially inner ends of the walls. The end plate is welded or brazed to the radially inner ends of the sidewalls of the serpentine passages combined by the root turn.